The present invention relates generally to apparatus and methods for treating medical conditions, and more specifically, to stents and stent-grafts for use in body vessels to treat those medical conditions.
Stents may be inserted into an anatomical vessel or duct for various purposes. Stents may maintain or restore patency in a formerly blocked or constricted passageway, for example, following a balloon angioplasty procedure. Other stents may be used for different procedures, for example, stents may be used a part of a “stent-graft,” whereby one or more stents are placed in or about a graft and used to hold the graft in an open configuration to treat an aneurysm or other condition. Additionally, stents coupled to one or both ends of a graft may extend proximally or distally away from the graft to engage a healthy portion of a vessel wall away from a diseased portion of an aneurysm to provide endovascular graft fixation.
Stents may be either self-expanding or balloon-expandable, or they can have characteristics of both types of stents. Self-expanding stents may be delivered to a target site in a compressed configuration and subsequently expanded by removing a delivery sheath, removing trigger wires and/or releasing diameter reducing ties. With self-expanding stents, the stents expand primarily based on their own expansive force without the need for further mechanical expansion. In a stent made of a shape-memory alloy such as nitinol, the shape-memory alloy may be employed to cause the stent to return to a predetermined configuration upon removal of the sheath or other device maintaining the stent in its predeployment configuration.
When a stent-graft having at least one stent is deployed in a vessel, such as the aorta, and blood flows in a proximal to distal direction away from the heart, there is a possibility of “infolding” of graft material, particularly at the proximal end of the graft material. For example, if a stent-graft is deployed to treat an abdominal aortic aneurysm, blood flowing distally into the graft may pull the proximal edge of the graft in a radially inward direction, particularly if an optimal proximal seal is not achieved with the vessel wall. In this case, the graft material that becomes pulled inward may impede blood flow through the stent-graft lumen, or an endoleak may occur. Furthermore, if the proximal end of a stent-graft is deployed in a curved portion of a vessel, such as the aortic arch or thoracic aorta, it may be difficult to conform the proximal edge of the stent-graft to the curving vessel wall, which also may result in blood flow catching on the graft and potential endoleaks.